Method for producing iron oxide pigments from waste acid resulting from TiO2 production

ABSTRACT

Process for the preparation of iron oxide pigments from the waste acid resulting from the preparation of titanium dioxide by the sulfate process, characterized in that in a first stage a partial neutralization of the sulfuric acid contained in the waste acid is performed with compounds from the group comprising metallic iron and/or iron compounds, the sulfuric acid is optionally further neutralized with a further alkaline compound, the precipitate containing Ti, Al, Cr and V compounds is separated from the resultant reaction product and an iron oxide yellow pigment or iron oxide black pigment is precipitated from the resultant iron sulfate-containing solution by addition of alkaline compounds as well as an oxidizing agent, each pigment being able to be baked to form an iron oxide red pigment.

BACKGROUND OF THE INVENTION

The present invention relates to a process for the preparation of ironoxide pigments from waste acid occurring in the production of titaniumdioxide by the sulfate process.

The preparation of iron oxide pigments from residues resulting fromtitanium dioxide production has already been known for a fairly longtime, for example from DE-A 24 60 493 and JP-A 44 050 695. Inparticular, the iron sulfate heptahydrate (green salt) occurring intitanium dioxide production is used as a raw material for preparing ironoxide pigments. The typical procedure involved in this process isdescribed for example in Ullmann's Encyclopedia of Industrial Chemistry,5th ed., Vol. A 20, p. 297 et seq.

Waste acid, which also occurs in the production of titanium dioxide bythe sulfate process, is however less suitable for preparing iron oxidepigments. Although iron oxides can also be obtained from waste acid in asimilar way to processes normally used for the preparation of iron oxidepigments, the quality of these iron oxides falls far short of therequirements that high quality iron oxide pigments, in particular ironoxide red pigments or iron oxide yellow pigments, have to satisfy. Thereason for this are undesirable constituents of the waste acid, such ascompounds of Ti, Cr, V or Mn, which adversely affect the particle shape,particle size, stability and colour properties of the iron oxidepigments obtained therefrom. In addition the large amount of freesulfuric acid in the waste acid, which first of all has to beneutralised for the preparation of iron oxide pigments, adverselyaffects the profitability of the process.

The utilisation or at least harmless disposal of the waste acid isprescribed in Europe and in most other countries that produce titaniumdioxide, with the result that various processes for utilisation havebeen developed over the course of time.

Printed specification EP-A 577 272 describes how a utilisable gypsum canbe obtained from the waste acid by partial neutralisation with calciumcarbonate. The metal sulfate solution remaining after separation of thismaterial, which is also known as “white gypsum”, is adjusted to a pH ofapproximately 9 by addition of CaO or CaCO₃, the material obtainedthereby, which is also known as “red gypsum”, then having to be disposedof. On the one hand valuable raw materials are obtained from arecycling, but on the other hand a large amount of valuable disposalspace is used up since, depending on the titanium raw material, 1 to 2.5tonnes of this waste is formed per tonne of TiO₂ pigment that isproduced.

Another process for working up the waste acid, described in EP-A 0 133505, avoids these disadvantages by reusing practically all the occurringwaste acid for the preparation of TiO₂, wherein the waste acid is firstof all concentrated and, after separating the filtration salts that areformed, the 65% to 85% sulfuric acid is reused to digest the ore. Sulfurdioxide is obtained from the filtration salts by thermal cracking andfrom this SO₂ pure sulfuric acid or oleum is obtained, which likewiseare reused for digesting the ore. Although this process minimises theconsumption of raw materials, it is very energy-intensive and thereforeexpensive to operate.

A further process for working up the waste acid, which is described inprinted specification U.S. Pat. No. 3,016,286, involves theneutralisation of the waste acid and the precipitation and separation ofhydroxides of Ti, Al, Cr and V, as well as the subsequent precipitationof magnetite with ammonia. The disadvantages of this process are howeverthe fact that in the first place large amounts of ammonia are consumedin neutralising the free sulfuric acid, and the magnetite precipitatedfrom the highly concentrated ammonium sulfate solution does not exhibitany pigment properties.

In a modification of the process described in printed specification EP-A638 515 involving an extraction of magnesium from the ammoniumsulfate-containing solution, pure ammonium sulfate can then be obtainedby crystallisation and used as fertiliser. Although the amount ofutilisable material is increased, the profitability of the process ishowever unsatisfactory on account of the additional process steps andthe poor quality of the magnetite obtained from the highly concentratedammonium sulfate solution.

Another variant for working up the waste acid comprises, according toU.S. Pat. No. 4,137,292 and DE-A 24 56 320, precipitating gypsum andmagnetite simultaneously by neutralising the waste acid with calciumcompounds, wherein in order to be able to utilise the two compounds amechanical separation has to be carried out, for example by means of ahydrocyclone or by magnetic separation, which however provides neitherpure gypsum nor a pure magnetite pigment despite the complicated andcostly process steps. An optimisation of the process according to GB-A 1421 773 is based on the fact that ammonium or alkali metal salts aresimultaneously present in the precipitation of gypsum with calciumcompounds, though this modification also does not avoid theaforementioned principal disadvantages of this process.

The presence of relatively large amounts of manganese in the iron saltsolutions interferes in the preparation of iron oxide yellow or ironoxide red pigments. If the starting materials α-FeOOH or magnetite thatare used for the preparation of iron oxide red contain more than 0.11wt. % manganese, referred to iron, then no high grade red pigment can beobtained. The presence of Cr, V, Ti and other chromophoric metal ions ormetal ions affecting the precipitation process likewise interfere.

The object of the invention is accordingly to develop a process thatenables the occurring waste acid to be processed so as to producequalitatively high grade iron oxide pigments.

It has now surprisingly been found that high quality iron oxide pigmentscan be prepared from the waste acid resulting from the production oftitanium dioxide by the sulfate process, that a partial neutralisationof the free sulfuric acid contained in the waste acid is carried outwith metallic iron and/or iron compounds, the undesired elements Ti, Al,Cr and V are at least partially precipitated as hydroxides andseparated, and the iron sulfate-containing solution obtained after theirseparation is processed further into iron oxide pigments.

SUMMARY OF THE INVENTION

The invention accordingly provides a process for the preparation of ironoxide pigments from the waste acid resulting from the production oftitanium dioxide by the sulfate process, characterised in that in afirst stage a partial neutralisation of the sulfuric acid contained inthe waste acid is carried out with compounds from the group comprisingmetallic iron and/or iron compounds, the sulfuric acid is optionallyneutralised further with a further alkaline compound, the precipitatecontaining the Ti, Al, Cr and V compounds is separated from theresultant reaction product, and an iron oxide pigment is precipitatedfrom the resultant iron sulfate-containing solution by adding alkalinecompounds as well as an oxidizing agent.

DETAILED DESCRIPTION OF THE INVENTION

The undesirable elements Ti, Al, Cr and V are precipitated either duringthe addition of the metallic iron or of the iron compounds if this stageis carried out at a sufficiently high pH; the increase in the pHnecessary to precipitate the undesirable elements may however also beachieved after completing the reaction with iron or iron compounds, byadding other alkaline compounds.

The iron sulfate-containing solution obtained after separating theundesirable elements may be converted either to a magnetite pigment,which can then be baked in a manner known per se to an iron oxide redpigment or iron oxide yellow pigment, or alternatively an iron oxideyellow pigment (α-FeOOH) can be prepared from the ironsulfate-containing solution also after the precipitation process, whichpigment can then be baked in a manner known per se to an iron oxide redpigment.

Metallic iron or basic iron compounds with a manganese content of <0.8wt. % Mn, referred to Fe, are preferably used to neutralise the wasteacid in the first stage. A manganese content of <0.4 wt. % Mn, referredto Fe, is particularly preferred. In this way particularly intenselycoloured and high grade iron oxide red pigments can be obtained evenfrom a waste acid that has a manganese content of 1 to 5 wt. % Mn,referred to Fe, in particular 1.5 to 2.5 wt. %.

The partial neutralisation of the sulfuric acid in the first stage ispreferably carried out by adding metallic iron, iron oxides or ironhydroxides. The partial neutralisation of the waste acid in the firststage may also be carried out by a mixture of various iron-containingsubstances, for example a mixture of metallic iron and iron oxidesand/or iron hydroxides. The process according to the invention ischaracterised in that, on account of the separation of the foreign metalhydroxides, even normally unsuitable iron waste materials that containCr, V, Ni, Co or other undesirable impurities can be used as rawmaterial. It is particularly economical to use iron-containing residuesthat otherwise would have to be disposed of expensively, for exampleiron scrap, iron-containing production residues, for example ironoxide-containing residues from the Laux process, or mill scale, turningsor cast iron cuttings.

When using iron(III) compounds it has to be ensured that the Fe(III)content in the solution remains as low as possible, by simultaneousaddition of metallic iron.

The successive use of several different sources of iron having differentreactivities may be particularly favourable. For example, theneutralisation of the waste acid is preferably first of all performedwith a less reactive material such as mill scale, and the furtherneutralisation of the now partially neutralised waste acid is thencarried out with more reactive materials such as cast iron turnings.

Also, inert or insoluble constituents in the iron materials do notinterfere since these can be separated and disposed of in the followingstage together with the compounds precipitated in this stage.

It is advantageous to dilute the waste acid before the reaction with theiron compounds in order to reduce the viscosity of the reaction mixtureand retain the resultant iron sulfate in solution. Either fresh water ora process water that occurs in the further course of the process can beused for the dilution.

The partial neutralisation of the sulfuric acid in the first stage withiron or iron-containing compounds is carried out at a pH of 0.5 to 4.7,preferably at a pH of 2.5 to 4.7. The solution is then optionallyfurther neutralised to a pH of 3.0 to 5.0, preferably to a pH of 4.0 to4.8, by the addition of further alkaline compounds.

In one embodiment of the present invention the neutralisation of thesulfuric acid in the first stage is carried out in two steps, preferablywith two iron materials of different reactivity, whereas in the firststep the less reactive iron material is added in a pH range of 0.5 to3.5 and in the second step the more reactive iron material is added in apH range 2.5-4.7.

The precipitate of the hydroxides of Ti, Al, Cr, V and Fe(III) obtainedin this way is separated from the liquid phase and can either bedisposed of or used as raw material for obtaining Ti, Al, Cr or V.Undissolved constituents originating from the raw materials that areused are optionally separated at this point together with thehydroxides. Fe(III) that may possibly be present before theprecipitation of the Ti-, Al-, Cr- and V-containing precipitate ispreferably reduced to Fe(II) by adding a reducing agent, in particularmetallic iron. In this way the amount of precipitate from the secondprecipitation stage that has to be disposed of or utilised further isreduced and the yield of high grade iron oxide pigment in the followingstage is increased. In the reduction of the Fe(III) the reaction shouldhowever be controlled so that as little Ti³⁺ as possible is formed,which would then not be precipitated so well as Ti⁴⁺. The precipitationof the titanium can if necessary be improved by adding crystallisationseeds of hydrated titanium oxide.

If for the neutralisation of the waste acid further alkaline compoundsare also used in addition to metallic iron or iron compounds, it isrecommended to use compounds that form easily filtrable sparinglysoluble sulfates, for example CaO or Ca(OH)₂, so that the filterabilityof all the resultant solids is significantly improved. The use of powerstation ash, refuse incineration ash or another alkaline-reacting ash asneutralising agent is particularly preferred.

The iron sulfate-containing solution obtained after separation of theTi-, Al-, Cr- and V-containing solids preferably has a manganese contentof <0.9 wt. %, particularly preferably of <0.5 wt. %, referred to Fe.

Iron oxide yellow pigments (α-FeOOH, Goethite) and iron oxide blackpigments (Fe₃O₄, magnetite) can be prepared in a manner known per se bythe precipitation process, from the iron sulfate-containing solutionobtained after separation of the solids (Ullmanns Encyclopedia ofIndustrial Chemistry, 5th ed., Vol. A 20, p. 297 et seq). Although it ispossible to prepare iron oxide red by the precipitation process, it issubstantially more difficult.

For the precipitation of the iron oxide, the iron sulfate-containingsolution is preferably adjusted to a concentration of 150 to 250 g,particularly preferably to 180 to 190 g FeSO₄ per litre. Thisconcentration can optionally be adjusted by evaporation or dilution.This adjusted iron sulfate-containing solution is converted in a mannerknown per se with the addition of alkaline compounds and an oxidizingagent to form a magnetite pigment or iron oxide yellow pigment.

To precipitate the iron oxide yellow pigment (α-FeOOH) preferably 4 to30 times the amount (calculated as Fe) of FeSO₄ is added in the form anaqueous solution to an α-FeOOH seed prepared in a manner known per se(Ullmann's Encyclopedia of Industrial Chemistry, 5th ed., Vol. A20, p.29⁷ et seq), and heated to a temperature of between 60° and 85° C. whilestirring. Oxidation is then carried out with an oxidizing agent whilethe pH value of the solution is adjusted to an end pH of 3.0 to 5.0 at arate of 0.01 to 0.4 pH units/hour. In addition alkaline-reactingcompounds that do not form sparingly soluble sulfates, for examplegaseous NH₃ or NH₃ dissolved in water, NaOH, KOH, MgO or Mg(OH)₂, areadded as precipitating agent. As oxidizing agent there may for examplebe used compounds from the group comprising oxygen, ozone, H₂O₂, sodiumhypochlorite, chloride of soda, chlorates, perchlorates, nitrates andchlorine. Oxygen or an oxygen-containing gaseous mixture, in particularair, is preferably passed into the reaction mixture. The oxidation ispreferably terminated as soon as the Fe-II content of the suspension isless than 1 mole-%.

Modifiers that control the particle shape and the particle sizedistribution may be added during the pigment formation. Aluminiumcompounds and zinc compounds as well as phosphates are particularlyeffective for this purpose. Organic modifiers such as aliphatic amines,hydroxycarboxylic acids, aliphatic alcohols or carboxylic acids or theirderivatives may however also be used.

The pigment suspensions are worked up by means of the known steps offiltration, drying and grinding. By suitably varying the preparationconditions, the person skilled in the art is able to prepare a broadrange of iron oxide yellow pigments of various particle sizes and thusvarious shades.

For the preparation according to the invention of an iron oxide blackpigment (magnetite) an amount of a precipitating agent is preferablyadded to the iron sulfate-containing solution so that the ratio ofiron(II) to precipitating agent is 0.4 to 0.65, particularly preferably0.5 to 0.58 equivalents. For this purpose preferably the calculatedamount of the precipitating agent is pumped into the ironsulfate-containing solution at a temperature between 60° and 95° C., inparticular between 75° and 95° C. Alkaline-reacting compounds that donot form sparingly soluble sulfates, for example gaseous NH₃ or NH₃dissolved in water, NaOH, KOH, MgO or Mg(OH)₂, are used as precipitatingagent. Compounds for example from the group comprising oxygen, ozone,H₂O₂, sodium hypochlorite, chloride of soda, chlorates, perchlorates,nitrates and chlorine are then added as oxidizing agent. Preferablyoxygen or an oxygen-containing gaseous mixture, in particular air, ispassed into the reaction mixture. The pH of the solution is preferablymaintained substantially constant. The oxidation is preferablyterminated as soon as the Fe(II) content of the suspension is less than1 mole-%.

The pigment suspensions are worked up by means of the known steps offiltration, drying and grinding. By suitably varying the preparationconditions, the person skilled in the art is able to prepare a broadrange of iron oxide black pigments of various particle sizes and thusvarious shades and stability.

By means of the aforedescribed processes it is possible to prepare fromwaste-acid α-FeOOH or Fe₃O₄ with a manganese content of ≦0.11 wt. % Mn,preferably ≦0.06 wt. % Mn, referred to iron. On account of their lowmanganese content these compounds are particularly suitable for bakingto form iron oxide red pigments.

To prepare iron oxide red pigments (Fe₂O₃) an α-FeOOH or Fe₃O₄ preparedaccording to the invention is calcined at temperatures between 350° and1000° C., particularly preferably between 600° and 900° C., in thepresence of an oxygen-containing gas. The implementation of the processaccording to the invention is not restricted to the use of a certaintype of furnace. The calcination may for example be carried out in afluidised bed furnace, a box furnace or a rotary kiln. The correspondingnecessary residence times should be matched to the furnace that is used.Following the calcination a grinding is normally necessary, especiallywhen calcination temperatures above 600° C. are employed. A dry grindingis preferably performed, with the addition of a conventional grindingaid, for example amines, alcohols or polyacrylates. Since the iron oxidered pigment prepared according to the invention does not on account ofits narrow particle size distribution and its low degree ofagglomeration require a particularly intensive grinding, a broad rangeof mills can be employed. Jet mills, impact mills, pendulum mills androller mills are suitable for grinding the red pigments obtainedaccording to the invention.

A preferred form of the process according to the invention comprisesusing ammonia in gaseous form or dissolved in water, to precipitate theiron oxide pigment. The ammonia can then be recovered wholly orpartially from the (NH₄)₂SO₄-containing solution obtained afterseparating the iron oxide pigment, by adding CaO. After addition of CaOor Ca(OH)₂ up to a pH value of approximately 10, ammonia is released ingaseous form and can be reused directly without compression or any othertreatment, for the iron oxide precipitation. Particularly preferably theresidual ammonium content is removed from the suspension from which thegaseous ammonia was obtained after the addition of the CaO or Ca(OH)₂,by stripping with steam or air in a stripping column. The aqueousammonia solution obtained in this way can be reused without furthertreatment in the iron oxide precipitation.

The gypsum-containing precipitate remaining after the ammonia recoverycan be filtered off and, after neutralisation, be utilised for exampleas a building material after washing with dilute sulfuric acid. Thealkaline filtrate contains essentially magnesium sulfate and can be usedas fertiliser or, after neutralisation, discharged as waste watercontaining neutral salts.

Alternatively the ammonia can be released by adding barium oxide,hydroxide or carbonate, with the formation of barium sulfate.

The iron oxide pigments obtained according to the invention haveparticularly good pigment optical properties since the ironsulfate-containing starting solution used for this purpose contains, onaccount of the preliminary precipitation of the metal hydroxides,particularly small amounts of heavy metals, in particular small amountsof Cr and V. By means of the process according to the invention it ispossible to process the waste acid into high grade pigments using simpleindustrial operations and with a comparatively low energy expenditure,and to reduce the amount of waste to a minimum, preferably toapproximately 0.2 to 0.7 tonne of disposable waste per tonne of producedTiO₂. Furthermore the process according to the invention even enablesindustrial residues to be used efficiently to prepare qualitatively highgrade iron oxide pigments, the profitability of the process therebybeing considerably improved. Of particular economic importance is theability to be able to use relatively unreactive or impureiron-containing materials that cannot be employed, or only with greateffort and expense, in other production processes for producing ironoxide pigments.

The iron oxide pigments obtained according to the invention may be usedto colour paints, lacquers, plastics, building and constructionmaterials, paper or other materials. The magnetite obtained according tothe invention may furthermore also be used as a magnetic pigment fortoners.

The shade of the iron oxide pigment that is obtained is determinedaccording to the following procedure:

Measurement of the pure shade iron oxide pigments:

The pigment is dispersed using a muller (plate-type colour dispersingmachine) in an air-drying lacquer system. The lacquer system (lacquer)consists of the following components:

95.26%  ® ALKYDAL F 48 binder, Bayer AG, middle oil, air-drying alkydresin based on drying vegetable fatty acids in a 38:7 mixture of whitespirit/xylene with a non-volatile fraction of ca. 55%, oilcontent/tri-glyceride in the non-volatile fraction ca. 48%, phthalicanhydride in the non-volatile fraction ca. 26%) 0.78% 2-butanone oxime,55% in white spirit (anti-skinning agent) 1.30% ® Octa Soligen Calcium(wetting agent, calcium salt of branched C₆-C₁₉ fatty acids in ahydrocarbon mixture (containing 4% Ca), Borchers AG) 0.22% ® OctaSoligen Cobalt 6 (drying agent, cobalt(2+) salt of branched C₆-C₁₉ fattyacids in a hydrocarbon mixture (containing 6% Co), Borchers AG) 0.87%® Octa Soligen Zirconium 6 (drying agent, zirconium salt of branchedC₆-C₁₉ fatty acids in a hydrocarbon mixture (containing 6% Zr), BorchersAG) 1.57% Glycolic acid n-butyl ester (= butyl hydroxyacetate) (flowimprover).

The components are mixed in a high-speed stirrer to form the finishedlacquer. A plate-type colour dispersing machine (muller) is used, asdescribed in DIN EN ISO 8780-5 (April 1995). An ®ENGELSMANN JEL 25/53muller with an effective plate diameter of 24 cm is used. The rotationalspeed of the lower plate is ca. 75 min⁻¹. By suspending a 2.5 kg weighton the loading stirrup the force between the plates can be adjusted toca. 0.5 kN. 0.8 g of pigment and 2.00 g of lacquer are dispersed in onestage at 100 revolutions using a 2.5 kg weight according to the processdescribed in DIN EN ISO 8780-5 (April 1995) Section 8.1. The muller isopened an d the lacquer is quickly collected on the lower plate outsidethe centre point. A further 2.00 g of lacquer are then added and theplates are brought together. The preparation is finished after twofurther stages each of 50 revolutions and without any load.

The pigmented lacquer is applied using a film coater (gap width at least150 μm, at most 250 μm) on a non-absorbent paperboard. The coatedpaperboard (lacquer coat) is then dried for at least 12 hours at roomtemperature. Before the colour measurement the lacquer coat is dried andcooled for one hour at ca. 65° C. (±5° C.).

Measurement of the optical brightening of iron oxide pigments:

The pigment and the optical whitener are dispersed using a muller (platetype colour dispersing machine) in an air-drying lacquer system. Acommercially available ®Bayertitan R-KB-2 titanium dioxide pigment(Bayer AG) is used as optical whitener. This pigment corresponds to TypeR 2 in ISO 591-1977. The lacquer system (lacquer) corresponds to thatused to determine the pure shade (see above).

The components of the lacquer system are mixed in a high-speed stirrerto produce the finished lacquer.

The pigmented lacquer and the lacquer coat are produced as described forthe measurement of tile pure shade (see above), 0.1500 g of the pigmentto be tested, 0.7500 g of Bayertitan R-KB-2 and 2.00 g of lacquer beingweighed out.

Colour measuring instrument:

A spectrophotometer (“colour measuring instrument”) with an Ulbrichtglobe having a measuring geometry of d/8 without a gloss trap is usedfor the measurements. This measuring geometry is described in ISO7724/2-1984 (E) Point 4.1.1, in DIN 5033 Part 7 (July 1983) Point 3.2.4and in DIN 53 236 (January 1983) Point 7.1.1. A ®Dataflash 2000measuring instrument of Datacolor International is used for themeasurements.

The colour measuring instrument is calibrated against a white, ceramicwork standard as described in ISO 7724/2-1984 (E) Point 8.3. Thereflection data of the work standard against an ideally matt-white bodyare fed into the colour measuring instrument so that, after calibrationwith the white work standard, all colour measurements are referred tothe ideally matt-white body. The black point calibration is performedwith a black hollow body supplied by the manufacturer of the colourmeasuring instrument.

Colour measurement:

Any gloss trap present is disconnected. The temperature of the colourmeasuring instrument and test piece is ca. 25° C.±5° C.

The lacquer coat is applied to the colour measuring instrument in such away that the measurement opening covers a central point of the lacquerlayer. The coat must be. applied fully and smoothly. The measurementopening must be completely covered by the lacquer layer. The measurementis then performed.

Calculation of the CIE coordinates:

The CIE coordinates L*, a* and b* of 1976 are calculated from themeasured reflection spectrum according to the calculation instructionsgiven in ASTM E 308-1985, Point 7. The weighting functions of thestandard illuminant C and of the 2°-standard observer of 1931 in ASTM E308-1985, Table 5.6, are employed. The wavelength range is between 400nm and 700 nm. The wavelength interval is 20 nm. No gloss is deducted inthe calculations. The remission values obtained are converted accordingto DIN 5033, Part 3 (July 1992) to the CIELAB colour data system.

The relative colouring strength is calculated in a similar way to therelative scattering capacity according to DIN 53 165 (Point 3.4) usingBayertitan R-KB-2 as optical whitener and a suitable Bayferrox referencepigment (instead of carbon black). ρ is used as the standard colourvalue Y/100.

The invention is described hereinafter by way of example, though thisshould not be regarded as restricting the scope of the invention. Theparts and percentages given in the examples refer to parts andpercentages by weight, unless otherwise specified.

EXAMPLE 1 Preparation of the Iron Sulfate-containing Solution for thePrecipitation of Iron Oxide Pigments

46.6 g of cast iron turnings (composition see below) are added at 80° C.during 170 minutes to 250 g of waste acid (composition see below) with amanganese content of 2.05% Mn, referred to Fe. The pH value of thesolution rises to 3.6. CaO is then added up to a pH of 4.5. Afterdiluting with 350 g of water and separating the precipitate byfiltration, 549 g of clear, green solution is obtained with a FeSO₄content of 17.6% and a manganese content of 0.48% Mn referred to Fe. TheFe content of the solution is 6.46%. In addition 37.7 g of filter cake(dry) are obtained. The solution is adjusted up to the furtherprocessing with a small amount of sulfuric acid to a pH of 2, in orderto avoid oxidation of the Fe(II).

Waste acid Cast iron turnings FeSO₄ solution pH  <0 —  4.4 H₂SO₄ 24.2% —— Fe 2.98% 91.1%  6.46% Ti 0.38% <0.001% Al 0.31% 0.28%   0.002% Cr0.030%  0.006%  <0.001% Mn 0.061%  0.09%   0.031% Mg 0.64% 0.22%  0.245% Mn/Fe 0.0205   0.00099   0.0048 

EXAMPLE 2 Preparation of the Iron Sulfate-containing Solution for thePrecipitation of Iron Oxide Pigment

80 g of cast iron turnings (composition see Example 1) are added at 80°C. during 170 minutes to 250 g of waste acid (composition as inExample 1) with a manganese content of 2.05% Mn, referred to Fe. The pHvalue of the solution rises to 4.4. After diluting with water andseparating the precipitate by centrifugation, a clear, green FeSO₄solution is obtained having a manganese content of 0.49% Mn, referred toFe.

EXAMPLE 3 Preparation of the Iron Sulfate-containing Solution for thePrecipitation of Iron Oxide Pigment

250 g of waste acid (composition as Example 1) with a manganese contentof 2.05% Mn, referred to Fe, is reacted at 80° C. for 4 hrs with 50 g ofmill scale with a manganese content of 1.9% Mn. The pH value of thesolution rises to 1.1. Cast iron turnings (composition as in Example 1)are then added till the pH of the solution is 4.4. After diluting withwater and separating the precipitate by filtration, a clear, green FeSO₄solution is obtained.

EXAMPLE 4 Preparation of Iron Oxide Black Pigment (Magnetite)

2.864 kg of the iron sulfate solution obtained according to Example 1(during the intermediate storage adjusted to pH <2 with sulfuric acid inorder to prevent oxidation) is diluted with 2.673 kg of water, placed ina vessel provided with a stirrer and gassing device, and heated to 85°C. 0.66 kg of a 25% caustic soda solution (318 kg NaOH/l) is next addedin order to adjust the pH value of the reaction mixture to 7.0. Air isthen gassed in at a rate of 0.1 m³/hr until there is a potential jump inthe solution from ca. −700mV to approximately −200mV (afterapproximately 4.5 hr); the pH moves during the gassing to a value ofapproximately 4.5. The precipitated magnetite is suction filtered andwashed with 0.50 kg of water. 5.479 kg of filtrate, 0.52 kg of washfiltrate and 0.302 kg of filter cake with a solids content of 64.9%(heating at 60° C. to constant weight) are obtained. After drying thefilter cake in a circulating air drying cabinet at 60° C. anddeagglomeration with a hammer bar mill, 0.196 kg of magnetite isobtained having the following properties:

Ca: 0.002% SO_(4:)  2.41% Fe:  68.8% Ti: 0.002% Mg: 0.005% Mn: 0.039%Cr: <0.001%   V: 0.001% Al: 0.021% Na: 0.054% Mn/Fe: 0.00057   BET: 7.4m²/g Pure shade: L* = 12.4 a* = 0.8 b* = −0.4 ΔL* = −0.3 (againstBayferrox 330) Δa* = −0.1 (against Bayferrox 330) Δb* = −1.3 (againstBayferrox 330) Optical brightening: L* = 54.8 a* = 0.5 b* = −3.8 ΔL* =−1.9 (against Bayferrox 330) Δa* = −0.1 (against Bayferrox 330) Δb* =−0.5 (against Bayferrox 330) The relative tinting strength againstBayferrox 330 is 118%.

EXAMPLE 5 Preparation of Iron Oxide Yellow Pigment (α-FeOOH)

3.697 kg of the iron sulfate solution obtained according to Example 1(during the intermediate storage adjusted to pH <2 with sulfuric acid inorder to prevent oxidation) and 1.43 1 of industrially used (α-FeOOHseeds containing 32.5 g/l of FeOOH (Ullmann's Encyclopedia of IndustrialChemistry, 5th ed., Vol. A20, p. 297 et seq) are placed in a vesselequipped with stirrer and gassing device and heated to 85° C. Thesolution is then adjusted to a pH of 3.8 by adding caustic soda (318 gNaOH/l) continuously over 3 hrs, at the same time gassing with 33 l/h ofair. Gassing is then continued for a further 5 hrs, the pH value beingmaintained at 3.8 by addition of NAOH.

The precipitated iron oxide yellow pigment is suction filtered andwashed with water until the conductivity of the filtrate is <50 μS.

4.254 kg of filtrate, 2.407 kg of wash filtrate and 1.684 kg of filtercake with a solids content of 42.0% (heating at 60° C. to constantweight) are obtained. After drying the filter cake in a circulating airdrying cabinet at 60° C. and deagglomeration with a hanger bar mill,0.707 kg of iron oxide yellow pigment is obtained having the followingproperties:

Ca: 0.001% SO_(4:)  1.2% Fe:  61.3% Ti: 0.001% Mg: 0.003% Mn: 0.026% Cr:<0.001%   V: <0.001%   Al: 0.013% Na: 0.012% Mn/Fe: 0.0004  BET: 22.4m²/g Pure shade: L* = 62.7 a* = 7.8 b* = 45.6 ΔL* = 0.9 (againstBayferrox 1410 M) Δa* = −2.5 (against Bayferrox 1410 M) Δb* = −1.4(against Bayferrox 1410 M) Optical brightening: L* = 82.5 a* = 3.6 b* =38.2 ΔL* = 0.4 (against Bayferrox 1410 M) Δa* = −1.2 (against Bayferrox1410 M) Δb* = 0.0 (against Bayferrox 1410 M) The relative tintingstrength against Bayferrox 1410 M is 95%.

EXAMPLE 6 Calcination of the Iron Oxide Black Pigment

20 g of the magnetite obtained according to Example 4 are baked to forman iron oxide red pigment by heating the magnetite in a ceramic dish ina chamber furnace in an air stream of 600 l/h at a heating rate of 4°C./min and removing the dish from the furnace when the temperature is600° C.

After grinding for 60 seconds in a disintegrator mill an iron oxide redpigment is obtained having the following properties:

Pure shade: L* = 33.8 a* = 20.9 b* = 11.8 ΔL* = 2.0 (against Bayferrox180 M) Δa* = 2.4 (against Bayferrox 180 M) Δb* = 3.4 (against Bayferrox180 M) Optical brightening: L* = 63.7 a* = 15.7 b* = 3.8 ΔL* = −2.7(against Bayferrox 180 M) Δa* = 3.7 (against Bayferrox 180 M) Δb* = 3.6(against Bayferrox 180 M) The relative tinting strength againstBayferrox 180 M is 129%.

EXAMPLE 7 Calcination of the Iron Oxide Yellow Pigment

20 g of the iron oxide yellow pigment obtained according to Example 5are baked to form an iron oxide red pigment by heating the iron oxideyellow pigment in a ceramic dish in a chamber furnace in an air currentof 600 l/h at a heating rate of 4° C./min, and removing the dish fromthe furnace when the temperature reaches 600° C.

After grinding for 60 seconds in a disintegrator mill an iron oxide redpigment is obtained having the following properties:

Pure shade: L* = 41.7 a* = 26.2 b* = 22.1 ΔL* = 1.7 (against Bayferrox110 M) Δa* = −1.6 (against Bayferrox 110 M) Δb* = −0.6 (againstBayferrox 110 M) Optical brightening: L* = 59.9 a* = 27.0 b* = 22.4 ΔL*= 1.1 (against Bayferrox 110 M) Δa* = 0.6 (against Bayferrox 110 M) Δb*= 3.0 (against Bayferrox 110 M) The relative tinting strength againstBayferrox 110 M is 111%.

EXAMPLE 8 (Comparison Example)

20 kg of waste acid are introduced at 70° to 80° C. together with 5.00kg NH₃ to a stirred vessel and neutralised at a constant pH of 5.0. 0.38kg of a metal hydroxide-containing precipitate is obtained, which isseparated by filtration. The remaining solution (54 kg) containsapproximately 36% (NH₄)₂SO₄, 4.2% FeSO₄, 0.08% MnSO₄, 3.3% MgSO₄ andapproximately 56% of water.

This solution is oxidised in a second stage with ca. 2 m³ of air andmaintained at a pH of 7.0 during the oxidation by further addition of0.51 kg of NH₃. After filtration 1.14 kg of solids (magnetite) as wellas 53.5 kg of ammonium sulfate solution containing about 40.0%(NI)₂SO₄are obtained.

Characterisation according to the method described in the text providesthe following results:

Optical brightening: L* = 64.5 a* = 0.0 b* = −0.4 ΔL* = 1.6 (againstBayferrox 306) Δa* = 0.1 (against Bayferrox 306) Δb* = 0.4 (againstBayferrox 306) Bayferrox 306 is the black pigment with the lowestcolouring strength. Since the magnetite obtained from the highlyconcentrated ammonium sulfate solution has an even higher opticalbrightening and an even lower colouring strength, it is unsuitable as apigment.

EXAMPLE 9 Calcination of Iron Oxide Black Pigment (Comparison Example)

20 g of the magnetite obtained according to Example 8 are placed in aceramic dish and heated in a chamber furnace in an air stream of 600 l/hat a heating rate of 4° C./min and removed from the furnace when thetemperature reaches 600° C.

After grinding for 60 seconds in a disintegrator mill a product isobtained having the following properties:

Pure shade: L* = 28.8 a* = 5.4 b* = 3.8 ΔL* = −3.1 (against Bayferrox180 M) Δa* = −13.1 (against Bayferrox 180 M) Δb* = −4.6 (againstBayferrox 180 M) Optical brightening: L* = 81.6 a* = 23.7 b* = 2.3 ΔL* =15.1 (against Bayferrox 180 M) Δa* = −8.3 (against Bayferrox 180 M) Δb*= 2.0 (against Bayferrox 180 M) The relative tinting strength againstBayferrox 180 M is only 18%.

No iron oxide red pigment can be prepared from the iron sulfate solutionof Example 9. The colour of the resultant product is brown. Use of theproduct as a brown pigment is excluded on account of the poor tintingstrength.

What is claimed is:
 1. A process for the preparation of an iron oxidepigment from the waste acid resulting from the preparation of titaniumdioxide by the sulfate process comprising a) obtaining a precipitatecontaining at least one of Ti, Al, Cr and V compounds by partiallyneutralizing in a first stage the sulfuric acid contained in the wasteacid with metallic iron and/or an iron compound, wherein the metalliciron and the basic iron compounds have a manganese content of <0.8% byweight of Mn, based on Fe. b) optionally obtaining a precipitatecontaining at least one of Ti, Al, Cr and V compounds by furtherneutralizing the sulfuric acid with a further alkaline compound, c)separating the precipitate containing Ti, Al, Cr and/or V compounds fromthe resulting reaction product to form an iron sulfate containingsolution, and d) precipitating an iron oxide pigment from the Ironsulfate containing solution by adding an alkaline compound and anoxidizing agent.
 2. The process of claim 1 comprising carrying out theneutralization of the waste acid with metallic iron or an iron compoundat a pH of 0.5 to 4.7.
 3. The process of claim 1 comprising carrying outthe neutralization of the waste acid in the first stage successivelywith two or more iron-containing materials of different reactivity. 4.The process of claim 1 comprising carrying out the neutralization of thewaste acid in a first stage with mill scale at a pH of 0.5 to 3.5 andthen with cast iron turnings at a pH of 2.5 to 4.7.
 5. The process ofclaim 1 comprising carrying out step b) with alkaline compounds at a pHof 3.0 to 5.0.
 6. The process of claim 1 wherein the furtherneutralizing agent in the first stage are alkaline compounds that formsparingly soluble sulfates.
 7. The process of claim 1 wherein thefurther neutralizing agent in the first stage power station ash, refuseincineration ash or another alkaline reacting ash.
 8. The process ofclaim 1 comprising obtaining an iron sulfate-containing solution byseparating the Ti-, Al-, Cr- and V-containing precipitate and adjustingthe concentration of FeSO₄ to between 150 and 250 g per liter optionallyby evaporation or dilution.
 9. The process of claim 1 comprisingconverting the iron sulfate-containing solution to an iron oxide blackpigment by adding an alkaline compound that does not form sparinglysoluble sulfates and an oxidizing agent.
 10. The process of claim 1comprising converting the iron sulfate-containing solution to an ironoxide yellow pigment by adding an alkaline compound that does not formsparingly soluble sulfates and an oxidizing agent.
 11. The process ofclaim 1 comprising using gaseous NH₃ or NH₃ dissolved in water or NaOH,KOH, MgO or Mg(OH)₂ in step (d) for precipitating the iron oxidepigments.
 12. The process of claim 11 comprising using oxygen or anoxygen-containing gaseous mixture as the oxidizing agent.
 13. Theprocess of claim 1 wherein the iron oxide pigment has a manganesecontent of <0.11% by weight, based on Fe.
 14. The process of claim 9comprising baking the precipitated iron oxide black pigment to an ironoxide red pigment after separation, purification and drying.
 15. Theprocess of claim 10 comprising baking the precipitated iron oxide yellowpigment to an iron oxide red pigment after separation, purification anddrying.
 16. The process of claim 12 comprising using ammonia asneutralizing agent in step (d) and releasing it from the(NH₄)₂SO₄-containing solution obtained after separation of theprecipitated iron oxide pigment, by adding CaO and/or Ca(OH)₂.